PHOSPHATO COMPLEXES OF PLATINUM(II) - P-31 NMR AND KINETICS OF FORMATION AND ISOMERIZATION STUDIES

Ortho-, pyro-, and triphosphate anions form monodentate phosphato complexes with chloro(diethylenetriamine)platinum(II) chloride and various chelates with dichloro(ethylenediamine)platinum(II) in the pH range 2-11. Monodentate complexations largely proceed through the direct reaction with the PtCl(dien)+ for which bimolecular rate constants were estimated as 2.7 × 10-2, 1.5 × 10-2, and 1.2 × 10-2 M-1 s-1 for the ortho-, pyro-, and triphosphate reactions. Phosphorus-31 NMR spectra reveal coordination chemical shifts of 3-6 ppm for the phosphorus atoms of the bound phosphate groups. Only the monodentate (γ-triphosphato)platinum(II) complex was detected by 31P NMR spectroscopy, although this ligand is capable of coordinating through the β-phosphate group as well. Phosphorus-phosphorus coupling constants for Pt(dien)(H2P2O7) and Pt(dien)(H2P3O10)- complexes were found to be in the range 19-23 Hz. Two acidity constants were estimated as 9.8 × 10-5 and 1.5 × 10-10 for Pt(H2PO4)(dien)+ and 7.3 × 10-5 and 5.6 × 10-8 for Pt(H2P2O7)(dien) from the pH-dependent 31P chemical shift data. Chelation to dichloro(ethylenediamine)-platinum(II) by pyro-and triphosphate ligands is accomplished primarily through aquations of the platinum substrate. Bimolecular rate constants for the direct reactions with PtCl2(en) were estimated as 1.9 × 10-3 and 4.6 × 10-4 M-1 s-1 for the pyro-and triphosphate anions at pH 6.0. The triphosphate ligand forms both β,γ-and α,γ-linkage isomers; the six-membered chelate ring (β,γ-complex) formation dominates at lower pH (3-4), whereas the formation of the eight-membered chelate ring is favored in the pH range >6. Coordinated phosphate groups exhibit 4-12 ppm coordination chemical shifts, and phosphorus-phoshorus coupling constants of 19-22 Hz were observed for the triphosphato chelates. Acidity constants, 6.0 × 10-4 and 8.3 × 10-6, were calculated for the Pt(H2P2O7)(en) complex. Linkage isomerizations, β,γ → α,γ and α,γ → β,γ, for triphosphato complexes mainly proceed through an intramolecular mechanism for which first order rate constants were estimated as 2 × 10-3 and 3 × 10-3 s-1 at pH 3.5 and 7.7. © 1990, American Chemical Society. All rights reserved.